A service brake accelerator of this type is known, e.g., from U.S. Pat. No. 4,206,949, which shows a reference pressure chamber connected to a brake control valve which contains a charging device, with throttle, for the reference pressure chamber from the main air supply. Thus, the prior art service brake accelerator is connected only to appropriate brake control valves, e.g., of the multi-release type, or independently of brake control valves. In the known device, the first piston element controls only the first valve. The second piston element is connected to a push rod which terminates at a distance from the closure element of the shut-off valve, and thus can open the latter after opening of the second valve. Only this shut-off valve and the third nozzle are located in a vent channel between the main air supply and atmosphere. The piston elements in the known device are formed as sealing elements for the first and second valves.
A further service brake accelerator with a nozzle between the line chamber and reference pressure chamber is known from U.S. Pat. No. 4,145,090. Here, too, two piston elements formed as diaphragm pistons are provided, the one of these elements which separates the line chamber from the reference pressure chamber controlling a relatively versatile change-over valve. Depending on the stroke adjustment of this piston element, the change-over valve either disconnects a nozzled vent from other connections, or connects the vent with both the main air supply and simultaneously with a chamber acting upon the other piston, or upon disconnection from the main air supply, only with the latter chamber. The other piston element is spring-biased against a valve seat having chambers on either side, the chamber on one side being connected with the reference pressure chamber and the chamber on the other side being connected via a nozzle with atmosphere. This known service brake accelerator, as already pointed out, discloses a complicated, demanding valve structure. In further known embodiments of this service brake accelerator additional structure is required for further valve and nozzle components between chambers on either side of the second piston element.
U.S. Pat. No. 4,073,544 discloses a further service brake accelerator, which however requires three piston elements in the form of diaphragm pistons, as well as a supplemental valve device between the reference pressure chamber and the line chamber. The construction of this service brake accelerator is hence even more complicated and demanding than that of the just described service brake accelerator.